Monday, October 08, 2007

Coupled

Across the tropical Indo-Pacific region the spatial distribution of SSTs clearly represents a basin wide cold event meaning negative anomalies in all Nino regions. TAO buoy recent five-day averaged anomaly magnitudes are at least minus 2.5C ~0/120W with totals colder than 20C off the west coast of South America. These negative anomalies (~minus 4C at 120W/100m) extend to roughly 200m depth along the cold tongue east of the date line. Please read the latest CPC ENSO diagnostics discussion and the MEI discussion from ESRL/PSD for further details (links given in Appendix).

The west central Pacific Ocean remains warmer than average with positive anomalies ~plus 1-2C and totals in excess of 30C. The warmest waters were from about 0/150E extending southeast into the region of 18S/180. Overall the warm spatial horseshoe pattern of SSTs extending into the extratropics remains well pronounced. The latter is also a characteristic of a La-Nina. However, I think the magnitudes and spatial coverage of the positive anomalies particularly across the Southern Hemisphere (greater than 1C) are impressive. These anomalously warm waters of the west central Pacific Ocean are slowly drifting south with the seasonal cycle.

Similar to a year ago, the spatial pattern of an anomalously warm equatorial Indian Ocean, cool around Indonesia then very warm west central Pacific has also become very well defined during the past 4-6 weeks. I think some of the coolness around Indonesia is a response to enhanced southeasterly surface winds from the Southern Hemisphere responding to Indian Ocean convection, along with cold air surges from the southern extratropics. Impacts onto La-Nina from this “Indian Ocean dipole” are unclear. Around 1 December 2006 dominate Indian Ocean tropical forcing “stopped El-Nino in its tracks”. Stay tuned. The tropical Atlantic Ocean extending into the Caribbean remains warmer than normal. In fact, anomaly magnitudes across the central and eastern tropical Atlantic Ocean have increased to ~plus 1C with totals of at least 27-28C over the past few weeks.

Obviously I went into greater length in regard to global tropical SSTs than I intended. However, other ocean basins also impact the global circulation besides Nino 3.4 (as supported by several modeling studies). For instance, is the recent warming across the Atlantic the start of an “Atlantic El-Nino” for those waters???

The global circulation has been strongly coupled to La-Nina since about 1 September in terms of SST forcing, tropical convection and circulation response, stuck in GSDM Stage 1. Around that time there was poleward propagation of zonal mean anomalous easterly wind flow from the equatorial into the subtropical atmospheres of both hemispheres. Anomaly magnitudes have been ~5-10m/s at 200mb located zonally ~30N and 20S. These easterlies have supported poleward shifted and at times very intense subtropical anticyclones (having enhanced westerly wind flow on their poleward sides). Global relative atmospheric angular momentum (AAM) has persisted around 2 atmospheric momentum units (AMUs) below the R1 data climatology, and there has been little variation of the Global Wind Oscillation (GWO). Broadly there has been poleward AAM transport ~40N and 30S with weak forcing from the surface torques.

Full disk satellite imagery and other diagnostic monitoring tools present a strong signal of tropical convective forcing developing back to the west across the equatorial Indian Ocean. Three-day averaged OLR/A suggest this convection to be centered ~5N/75E, with anomaly magnitudes ~minus 50-90W/m**2. Strong suppression continues across Indonesia and the west central/northwest Pacific convection has weakened considerably. Per animations of daily mean upper tropospheric vector wind anomalies the persistent Eastern Hemisphere twin subtropical anticyclones have shifted west as a response to ~110E. Rossby wave energy dispersions from these anticyclones are starting a slight retrogression of circulation anomalies across the PNA sector as I type.

Now comes the punch line to all this, at least for today. In past postings I have mentioned there have been ~4 subseasonal variations of the Eastern Hemisphere tropical convective forcing since early June 2007. I think the Indian Ocean forcing is the initiation of another event, perhaps even a weak MJO (yes, you read it here first!). I base this speculation on seasonal cycle considerations (closer to the equator) and the very warm waters of the west central Pacific Ocean. However, if a MJO develops it is highly probable to be truncated to the Eastern Hemisphere and any enhanced rainfall may stay north of Indonesia. Finally, I will again repeat my past concerns of 2 regions of Eastern Hemisphere tropical forcing (Indian Ocean and west central Pacific) and the possibility for the west Pacific Ocean convection becoming the dominate area.

Finally, unlike the past several weeks a GWO signal may also be appearing as a response to the development of Indian Ocean forcing. Per ESRL/PSD R1 AAM plots updated through 6 October global tendency has dipped ~minus 30 Hadleys having contributions from the surface torques and possible eddy feedbacks from the extratropics (large anticyclones). In fact, per Tokyo Climate Center a rare sudden stratospheric warming (SSW) of the austral stratosphere may be in progress given very strong upward EP fluxes and 10-hPa temperatures recently soaring to minus 10C. I discussed this notion in my 21 September posting. The point is events like a SSW are linked to the dynamics involving atmosphere-ocean coupling and subseasonal variability.

At this time anomalous zonal mean westerly wind flow is present ~45-50N (~5-10m/s at 200mb) due to seasonal transition and La-Nina feedbacks. Focusing on the PNA sector, as the Eastern Hemisphere tropical forcing moves into the warm west central Pacific over the next few weeks (sooner??), it is probable for the western USA to get more strong full latitude troughs starting sometime week 2. Yes, many models are suggesting one or two weak wave breaking residual lows to move into the Rockies and Plains before than. Most week-2 ensemble means from global centers suggest a trough off the USA west coast-central states ridge with a trough around the east coast. I think these forecasts will not perform well. In fact, during ~week 3 full-latitude ridging around 140W into Alaska with strong anticylonically wave breaking lows across the southwest states may be probable should a brief GWO circuit toward Stage 2 occur. However, timing of circulation variations remains unclear particularly this time of year.

Much of the USA west coast is probable to be impacted with several troughs for at least the next couple of weeks. Yes, strong North Pacific jets leading to baroclinically amplifying west coast troughs can also happen during La-Nina years. Alaska should remain north of the storm track. The Rockies and Plains should become active with perhaps a couple of strong (unlike the earlier weak systems week 1) baroclinic synoptic-scale storms during week 2 possibly continuing into week 3. During week 3 some Arctic air (for this time of year) may become involved leading to snow events across the northern High Plains. Heavy precipitation and even autumn severe local storms may again focus on the central and northern Mississippi Valley into the Ohio Valley.

Western USA troughs may become stronger and more persistent, leading to an active southwest flow storm track across the Plains, as we progress through the colder part of boreal seasonal cycle. Ramifications should be understood. Given the recently observed pronounced tendency for wave breaking and hence slower moving lows across the southern Rockies, perhaps this particular La-Nina base will support some wetness across the Desert Southwest and southern High Plains going into winter. Even though there are some indications for a moderate La-Nina, I stress the importance of monitoring the west central equatorial and eventually the Tropical Southwest Pacific Ocean for persistent anomalously strong convection particularly starting later December into early 2008.

Please see the latest statements from the NOAA/NWS/Tropical Prediction Center for tropical cyclone statements. Anticyclonic wave breaking still rules across the North Atlantic.

The west central into the northwest Pacific Ocean is probable to have a respite from typhoons and heavy rainfall week 1, with activity increasing weeks 2-3. Impact areas still include the Philippines and Southeast Asia. Another area of concern for tropical cyclones is the Bay of Bengal including coastal sections, especially if the convection moves east. Much of Europe is probable to have active weather for at least the next couple of weeks.

Appendix

An experimental quasi-phase space plot of the GSDM utilizing a time series of normalized relative AAM tendency anomaly (Y-axis) and normalized relative AAM anomaly time series (X-axis) can be found at

We call the behavior of this plot the Global Wind Oscillation (GWO). While the intent of the GSDM is to extend current thinking beyond the MJO, the purpose of the GWO is to illustrate the non-oscillatory stochastically forced component of the GSDM.

These are probabilistic statements, and work is ongoing to quantify in future posts (for example, risk assessment maps, signal to noise ratio plots and shifts of probability for subseasonal variability). We hope that an opportunity will arise for us to have a dedicated web page effort to expedite more objectively, with rigor, thoroughness and verification. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will try to post another discussion ~13-14 October.